What happens if an extension doesn't have planning permission?

First establish whether permission was actually needed. Many extensions fall within permitted development, in which case nothing is wrong — and a Lawful Development Certificate can prove it formally. The problem only exists where the work exceeded what was allowed.

Where there is a genuine breach, councils usually start with a letter, not a bulldozer. Enforcement is discretionary: officers ask whether the development causes planning harm. Minor, invisible-from-the-street breaches are often invited to regularise through a retrospective application; harmful ones attract an enforcement notice with a deadline to alter or remove the work.

A retrospective application is judged exactly like a normal one — no penalty fee, no prejudice, but also no sympathy: if the scheme would have been refused before it was built, it will be refused after. Then the enforcement notice follows, and failing to comply with a notice is a criminal offence carrying unlimited fines.

Time can cure a breach. In England, development substantially completed before 25 April 2024 became immune from enforcement after 4 years; work completed after that date falls under the newer 10-year rule introduced by the Levelling-up and Regeneration Act. Immunity can be formalised with a Lawful Development Certificate.

The quieter costs bite even without enforcement: unauthorised work surfaces in buyers' surveys and solicitors' searches, complicates remortgaging and insurance claims, and typically knocks more off a sale than the permission would have cost. If you've inherited or built an unpermitted extension, getting the paperwork straight sooner is nearly always cheaper than later.